Freezing of random RNA

We study secondary structures of random RNA molecules by means of a renormalized field theory based on an expansion in the sequence disorder. We show that there is a continuous phase transition from a molten phase at higher temperatures to a low-temperature glass phase. The primary freezing occurs above the critical temperature, with local islands of stable folds forming within the molten phase. The size of these islands defines the correlation length of the transition. Our results include critical exponents at the transition and in the glass phase.     

Mechanical, vibrational, and dynamical properties of amorphous systems near jamming

Amorphous systems undergo the jamming transition when the density increases, temperature drops, or external shear stress decreases, as described by the jamming phase diagram which was proposed to unify different processes such as the glass transition, random close packing, and yielding under shear stress. At zero temperature and shear stress, the jamming transition occurs at a critical density at Point J. In this paper, we review recent studies of the material properties of marginally jammed solids and the glassy dynamics in the vicinity of Point J. As the only singular point in the jamming phase diagram, Point J exhibits special criticality in both mechanical and vibrational quantities. Dynamics approaching the glass transition in the vicinity of Point J show critical scalings, suggesting that the molecular glass transition and the colloidal glass transition are equivalent in the hard sphere limit. All these studies shed light on the long-standing puzzles of the glass transition and unusual properties of amorphous solids.     

Holographic phase transitions with fundamental matter

The holographic dual of a finite-temperature gauge theory with a small number of flavors typically contains D-brane probes in a black hole background. At low temperature, the branes sit outside the black hole and the meson spectrum is discrete and possesses a mass gap. As the temperature increases, the branes approach a critical solution. Eventually, they fall into the horizon and a phase transition occurs. In the new phase, the meson spectrum is continuous and gapless. At large Nc and large 't Hooft coupling, we show that this phase transition is always first order. In confining theories with heavy quarks, it occurs above the deconfinement transition for the glue.     

Describing Hydrodynamic Particle Removal from Surfaces Using the Particle Reynolds Number

The fundamental processes related to the removal of fine particles from surfaces in a hydrodynamic flow field are not adequately understood. A critical particle Reynolds number approach is proposed to assess these mechanisms for fine particles when surface roughness is small compared to particle diameter. At and above the critical particle Reynolds number, particle removal occurs, while below the critical value, particles remain attached to a surface. The system under consideration consists of glass particles adhering to a glass surface in laminar channel flow. Our results indicate rolling is the removal mechanism, which is in agreement with the literature. Theoretical results of the critical particle Reynolds number model for rolling removal are in general agreement with experimental data when particle size distribution, particle and surface roughness, and system Hamaker constant are taken into account.     

Aspects of the phase diagram in (P)NJL-like models

We discuss three applications of NJL- and PNJL-like models to assess aspects of the QCD phase diagram: First, we study the effect of mesonic correlations on the pressure below and above the finite temperature phase transition within a non-local PNJL model beyond the mean-field approximation. Second, we reconstruct the phase boundary of an NJL model from a Taylor expansion of the chiral susceptibility about μ=0 and compare the result with the exact phase boundary. Finally, we demonstrate the realization of the “non-standard scenario” for the critical surface in a three-flavor PNJL model with a μ-dependent determinant interaction.     

In situ X-ray observation of decomposition of hydrous aluminum silicate AlSiO3OH and aluminum oxide hydroxide d-AlOOH at high pressure and temperature

We clarified the stability limit of phase Egg, AlSiO₃OH, a candidate for water reservoir in the siliceous sediment of slabs in the transition zone conditions by in situ X-ray observation using high energy X-ray from synchrotron radiation source of SPring-8. Phase Egg is stable at least up to 1625 °C at 17 GPa. We observed decomposition of phase Egg into δ-AlOOH and stishovite at pressures greater than 23 GPa at temperatures below 1200 °C. No water release occurs associated with the decomposition. At temperatures above 1200 °C at 23 GPa, we observed decomposition of phase Egg into corundum+stishovite+fluid. We also determined the phase boundary of the decomposition reaction of δ-AlOOH to corumdum+fluid based on the in situ X-ray diffraction at high pressure and temperature.     

Thermal studies of the spin liquid state and analog to the He melting curve in a geometrically frustrated magnet

Gadolinium gallium garnet, Gd₃Ga₅O₁₂ (GGG) has an extraordinary low-temperature phase diagram. Although the Curie–Weiss temperature of GGG is −2 K, GGG shows no long-range order down to T0.4 K. At low temperatures GGG has a spin glass phase at low fields (0.1 T), a field-induced long-range ordered antiferromagnetic state at fields of between 0.7 and 1.3 T, and, at intermediate fields, an apparent spin-liquid state without long-range order. We have characterized the intermediate field (IF) state through heat capacity, thermal conductivity, and magnetocaloric measurements. Our results show a sharp high-field phase boundary of the thermal irreversibility of the spin glass phase of GGG implying that the intermediate field phase is distinct from the spin glass. The lower field boundary of the AFM phase is shown to have distinct minimum at T0.2 K, in analogy to the minimum in the melting curve of ⁴He. The existence of such a minimum is confirmed by measurements of the latent heat of the transition below that temperature.     

Investigation into the dielectric relaxation of vinylidene fluoride copolymers with hexafluoropropylene

Four relaxation processes and one ferroelectric-paraelectric phase transition are revealed in vinylidene fluoride-hexafluoropropylene copolymers with different ratios of the components in the temperature range from ?100 to 150°C. The relaxation process occurring at the lowest temperature is associated with the local mobility of the chains, whereas the relaxation process at a higher temperature is due to micro-Brownian motion of segments in the amorphous phase in the glass transition range. A smeared relaxor phase transition from the polar modification of the α phase of vinylidene fluoride units to the paraelectric phase is observed in the temperature range 50–70°C. At higher temperatures, there occurs an intensive relaxation process that can be attributed to space-charge relaxation or manifestation of the normal relaxation mode.     

The transfer matrix for a pure phase in the two-dimensional Ising model

The problem of diagonalizing the transfer matrix for the two dimensional Ising model with all boundary spins equal to +1 is solved by use of the spinor method. This provides a simple proof that the spontaneous magnetization is actually given by the well known formula for the long range order with torodial boundary conditions, and this means that the critical temperature is precisely that temperature above which the state is unique and below which it is non unique. An expression for the magnetization at finite distance from the boundary is also given, and a simple derivation of the formula for the surface tension between two coexisting phases is presented. Finally the relation between the degeneracy of the spectrum and the phase transition is discussed.     

Holographic superconductor in the analytic hairy black hole

We study the charged black hole of hyperbolic horizon with scalar hair (charged Martinez-Troncoso-Zanelli: CMTZ black hole) as a model of analytic hairy black hole for holographic superconductor. For this purpose, we investigate the second order phase transition between CMTZ and hyperbolic Reissner-Nordström-AdS (HRNAdS) black holes. However, this transition unlikely occurs. As an analytic treatment for holographic superconductor, we develop superconductor in the bulk and superfluidity on the boundary using the CMTZ black hole below the critical temperature. The presence of charge destroys the condensates around the zero temperature, which is in accord with the thermodynamic analysis of the CMTZ black hole.     

Influence of capillary condensation on the near-critical solvation force

We argue that in a fluid, or magnet, confined by adsorbing walls which favor liquid, or the (+) phase, the solvation (Casimir) force in the vicinity of the critical point is strongly influenced by capillary condensation which occurs below the bulk critical temperature T(c). At T slightly below and above T(c), a small bulk field h<0, which favors gas, or the (-) phase, leads to residual condensation and a solvation force which is much more attractive (at the same large wall separation) than that found exactly at the critical point. Our predictions are supported by results obtained from density-matrix renormalization-group calculations in a two-dimensional Ising strip subject to identical surface fields.     

Exact results for the Kardar-Parisi-Zhang equation with spatially correlated noise

We investigate the Kardar-Parisi-Zhang (KPZ) equation in d spatial dimensions with Gaussian spatially long-range correlated noise -- characterized by its second moment -- by means of dynamic field theory and the renormalization group. Using a stochastic Cole-Hopf transformation we derive exact exponents and scaling functions for the roughening transition and the smooth phase above the lower critical dimension . Below the lower critical dimension, there is a line marking the stability boundary between the short-range and long-range noise fixed points. For , the general structure of the renormalization-group equations fixes the values of the dynamic and roughness exponents exactly, whereas above , one has to rely on some perturbational techniques. We discuss the location of this stability boundary in light of the exact results derived in this paper, and from results known in the literature. In particular, we conjecture that there might be two qualitatively different strong-coupling phases above and below the lower critical dimension, respectively. Received 5 August 1998     

Phase transition in the collisionless damping regime for wave-particle interaction

Gibbs statistical mechanics is derived for the Hamiltonian system coupling a wave to N particles self-consistently. This identifies Landau damping with a regime where a second order phase transition occurs. For nonequilibrium initial data with warm particles, a critical initial wave intensity is found: above it, thermodynamics predicts a finite wave amplitude in the limit N-->infinity; below it, the equilibrium amplitude vanishes. Simulations support these predictions providing new insight into the long-time nonlinear fate of the wave due to Landau damping in plasmas.     

Crack propagation as a free boundary problem

A sharp interface model of crack propagation as a phase transition process is discussed. We develop a multipole expansion technique to solve this free boundary problem numerically. We obtain steady state solutions with a self-consistently selected propagation velocity and shape of the crack, provided that elastodynamic effects are taken into account. Also, we find a saturation of the steady state crack velocity below the Rayleigh speed, tip blunting with increasing driving force, and a tip splitting instability above a critical driving force.     

Freezing of glycerol-water mixtures under pressure

We investigated freezing of pure glycerol as well as glycerol-water (GW) mixtures with 3:1 and 3:2 volume fractions as a function of pressure in the 0-10?GPa range by ruby fluorescence spectroscopy and neutron scattering. We find that the glass transition pressure increases from 5.5?GPa for pure glycerol to 6.5?GPa for the 3:1?GW mixture, with unusually small pressure gradients above. For higher water concentrations close to 3:2, phase separation occurs above 2?GPa where most of the water is expelled in the form of ice VII. The results suggest that glycerol is able to effectively hydrogen bond not more than ≈2.5 H(2)O molecules per glycerol, which seems to support conclusions from molecular dynamics simulations. The data indicate that these fluids could become important as pressure transmitting media for neutron scattering in the 0-7?GPa range, including at low temperatures.     

Band Jahn-Teller instability and formation of valence bond solid in a mixed-valent spinel oxide LiRh2O4

We have synthesized a new spinel oxide LiRh2O4 with a mixed-valent configuration of Rh3+ and Rh4+. At room temperature, it is a paramagnetic metal, but on cooling, a metal-insulator transition occurs and a valence bond solid state is formed below 170 K. We argue that the formation of valence bond solid is promoted by a band Jahn-Teller transition at 230 K and the resultant confinement of t_{2g} holes within the xy band. The band Jahn-Teller instability is also responsible for the observed enhanced thermoelectric power in the orbital-disordered phase above 230 K.     

Static and dynamic heterogeneities in a model for irreversible gelation

We study the structure and the dynamics in the formation of irreversible gels by means of molecular dynamics simulation of a model system where the gelation transition is due to the random percolation of permanent bonds between neighboring particles. We analyze the heterogeneities of the dynamics in terms of the fluctuations of the self-intermediate scattering functions: in the sol phase close to the percolation threshold, we find that this dynamic susceptibility increases with the time until it reaches a plateau. At the gelation threshold this plateau scales as a function of the wave vector k as k(eta-2), with eta being related to the decay of the percolation pair connectedness function. At the lowest wave vector, approaching the gelation threshold it diverges with the same exponent gamma as the mean cluster size. These findings suggest an alternative way of measuring critical exponents in a system undergoing chemical gelation.     

Initial nucleation stages and properties of CuCl nanoparticles in glasses

The time variation and temperature dependence of a CuCl phase nucleation in a glass was studied by exciton spectroscopy. The phase formation kinetics at three temperatures was measured. A time delay in attaining a stationary rate of the new phase growth was observed at all temperatures, in agreement with the Zeldovich theory. The kinetic parameters of the CuCl phase formation were determined in the initial stage, when the critical nuclei possessing a zero surface energy (and an effective radius below 1.3 nm) appear in the glass matrix. The first-order phase transition in the new phase is 200 K below the melting temperature of CuCl single crystals. The temperature dependence of the CuCl phase nucleation rate reveals the second and third stages of the new phase formation. The activation energies for diffusion of the CuCl phase components in the glass matrix are determined.     

Crystalline morphology in polymer blends via competition with spinodal decomposition

Miscible polymer blends containing one crystallizable component and exhibiting liquid-liquid phase separation at elevated temperatures [lower critical solution temperature (LCST) behavior] offer an excellent possibility of controlling morphology and thus mechanical properties. For instance, if a homogeneous mixture of dissimilar polymers is allowed to undergo a rapid temperature jump from below LCST to above LCST, spinodal decomposition takes place and a highly interconnected two-phase morphology with uniform domain size (so-called modulated structure) develops. By quenching the phase-separated system below the glass transition temperature after an appropriate time of phase separation, one is able to fix this characteristic morphology [1]. By quenching the phase-separated blend below the melting point of the crystallizable component to different supercooling depths, it is possible to control the number of nuclei and thus the spherulite's size, creating more or less ordered structures.     

Superfluid-insulator transition in a moving system of interacting bosons

We analyze the stability of superfluid currents in a system of strongly interacting ultracold atoms in an optical lattice. We show that such a system undergoes a dynamic, irreversible phase transition at a critical phase gradient that depends on the interaction strength between atoms. At commensurate filling, the phase boundary continuously interpolates between the classical modulation instability of a weakly interacting condensate and the equilibrium quantum phase transition into a Mott insulator state at which the critical current vanishes. We argue that quantum fluctuations smear the transition boundary in low dimensional systems. Finally we discuss the implications to realistic experiments.